Question PC runs for a few seconds than shutdown and restart

Aug 2, 2019
15
2
15
help please guys
mb : h61m-p20 G3
gpu : Gtx 960 (inplugged)
cpu: i5 2500
psu: 770W
ram : 8gb ddr3
 
Aug 2, 2019
15
2
15
hello !
i've tried 3 differents mb's and 5 ram's sticks and 3 different cpu's but nothing is working
specs
cpu : i5 2500
gpu : GTX 960 (inplugged)
mb : i've tried this 3 mb's : h61m-p20 (G3) / h61m-p30(G3)/h61m-k
ram : 5 sticks of different mhz / 1333mhz / 1600mhz
psu : chinese 750w but it works and the psu fan is working
hdd : inpluged

please help guys i've tried all the possible ways